Conventionally, various proposals have been made on design modifications for ground electrodes of spark plugs and techniques for suppressing wear of electrodes of spark plugs in order to attain improvements in ignition performance and flame propagation (see, for example, Japanese Laid-Open Patent Publication No. 2008-204882 and Japanese Laid-Open Patent Publication No. 2007-265842).
In recent years, there is a tendency that the air-fuel ratio is often set leaner than the stoichiometric air-fuel ratio during vehicle driving so as to improve vehicle fuel efficiency and to conform with exhaust emission regulation which gets stricter year after year. For improvement of vehicle fuel efficiency and conformity with exhaust gas regulation, complete combustion of air-fuel mixture is required irrespective of its air-fuel ratio. This results in a need to improve ignition performance in an air-fuel ratio range leaner than the stoichiometric air-fuel ratio. It has thus been attempted to improve ignition performance e.g. by increasing the value (energy) of electric current applied to the spark plug to generate a larger spark at ignition and by increasing the time for energization of the spark plug.
With the increase of the spark size and the increase of the energization time, however, it becomes likely that blowing of sparks will occur. The degree of wear of the ground electrode base material increases with increase in the frequency of exposure to blowing of sparks. As a result, there arises the possibility of misfiring due to separation of a noble metal tip from the ground electrode, breakage of the ground electrode etc. In particular, the wear of a basal end portion of the ground electrode leads to breakage of the ground electrode so that the spark plug becomes unable to perform its function. In the case of protecting the ground electrode by simply applying a coating of noble metal etc. to the ground electrode, on the other hand, it becomes likely that abnormal combustion will occur. In the conventional arts, sufficient considerations are not given to these problems.
There has accordingly been a demand to provide a spark plug capable of suppressing wear of a base material of a ground electrode and suppressing abnormal combustion.
The present invention has been made to address the above-mentioned problems and can be embodied in the following aspects.